1. Field of the Invention
This invention relates to semiconductor packages and more particularly to a firing process for fabricating a multi-layer glass-metal package.
2. Related Applications
U.S. patent applications Ser. No. 175,536 (Johnson et al.) and Ser. No. 175,530 now U.S. Pat. No. 3,726,002, issued Apr. 10, 1973 (Greenstein et al.) assigned to the assignee of the present application and filed on the same day as the present application, Aug. 27, 1971 are directed to distinct inventions connected with the overall glass-metal package.
3. Brief Description of the Prior Art
As the integrated circuit technology advances towards large-scale integration and high performance circuits, it is necessary to provide interconnection electrical packaging which is compatible with the performance demands of the associated circuitry. Thus, the problems of signal delay, package impedance, and cross-talk are extremely critical. Known prior art packaging materials often do not possess appropriate controllable dielectric properties to accommodate high performance circuits in large-scale integration schemes.
A high quality glass-metal package offers one solution to the problem. By simply changing the specific glass composition, the various desired range of properties are readily controllable and selectable. It has relatively high strength, and its chemically inert and thermally stable properties are extremely adaptable for known upper surface chip bonding techniques.
Despite the desirability of employing glass as the dielectric insulating layer in multi-layer interconnection packages, known processes for fabricating a multi-level package are almost non-existent or result in such poor quality products that the advantages attributable to certain dielectric properties of the selected glass are defeated.
Generally, one basic problem in forming multi-level glass layers is traced to the formation of bubbles occurring during the firing step. One type of bubble results from the decomposition of organic materials present on the surface upon which the glass layer is being deposited. This decomposition causes evolution of gases that are trapped or absorbed by the upper glass layer. The consequence of this type of bubble formation is to create voids in the glass structure. During subsequent metal evaporation steps, these voids or openings often cause electrical shorting due to metallization forming in the voids or openings. Also, the voids trap extraneous material so as to further aggravate the contamination problem. Even if the bubble remains intact or does not break in the glass, its presence often destroys the planarity of the upper glass surface so as to impair subsequent processing operations, such as the photolithographic steps.
A second type of bubble or closed cell structure also creates problems in the formation of multi-layer glass modules. The gaseous ambient surrounding the glass during the firing step forms bubbles in the glass layer. A closed cell or bubble is formed as the glass layer enters the sintering phase. Sintering is that point at which the solid glass particles start to soften under the exposure to heat, and begin to join or coalesce with adjacent particles. At the sintering temperature, the glass is not capable of reflowing into a homogeneous body without the formation of bubbles. As adjacent solid glass particles (having random geometrical shapes) begin to join, a closed cavity is formed. Stated in another way, necks are grown between two adjacent glass particles, and then the necks continue to grow between other pairs of particles, and thus ultimately, between all particles. At this point, an interconnected network of sintered glass particles are formed with voids throughout the network. The existence of this network prevents the fabrication of high quality multi-layer glass-metal modules having the desired impedance and planarity characteristics.
The prior art in the general glass area has suggested that bubble-free glass layers can be formed by outdiffusing the bubbles at a very high temperature. Often, this outdiffusion occurs in different gaseous ambients. However, this approach is totally unsuitable for the present multi-layer glass-metal modules, because the relatively high temperatures required to outwardly diffuse the trapped bubbles in the glass layer would completely destroy and disrupt previously deposited metallization lines and vertical metallic interconnection studs existing within the glass body.
A high quality bubble-free structure is also theoretically achievable by firing the glass in a complete vacuum. However, this approach causes a considerable number of practical problems, particularly in large-scale manufacturing operations due to the unfeasibility of working in this ambient.
Sputtering of successive glass layers to form a multi-level glass-metal package is another possible approach. However, this technique gives rise to significant disadvantages from a process and structural standpoint. Firstly, the sputtering process does not lend itself to the practical fabrication of glass layers of any appreciable thickness, which are sometimes necessary in order to obtain the desired impedance characteristics for the particular package designed. Further, it is not workable to build up a plurality of glass layers having interposed metallization patterns, and yet maintain each of the individual metallization patterns in a single plane.